A conventional circuit breaker is constructed as shown in FIG. 17, in which a current flows from a terminal block 71 to terminal 77 through a stationary contact 72, a movable contact 73, a movable arm 74, a bimetal 75 and a stranded wire 76 in that order. A handle 78, as shown in FIG. 17, it tilted leftwardly and contacts 72 and 73 are in contact with each other by a link 79, a hook segment 80 and the movable arm 74, so as to be well-balanced. In this state, when a overload current flows in the circuit, the bimetal 75 curves downwardly to push down a movable magnetic plate 81 and release it from the hook segment 80, whereby the movable arm 74, link 79 and hook segment 80 are unbalanced and a spring 82 turns the movable arm 74 in the reverse direction around a shaft 83 to disconnect the contacts 72 and 73. When a short-circuit current flows, the magnetic flux generated around the bimetal 75 instantaneously attracts the movable magnetic plate 81 toward the stationary magnetic plate 84 so that the hook segment 80 disengages from the movable magnetic plate 81, thereby disconnecting the contacts 72 and 73 the same as the above. In addition, in FIG. 17, reference numeral 85 designates a tension spring interposed between the movable magnetic plate 81 and the movable arm 74, which is disposed around a stem 86 and biases the free end of movable magnetic plate 81 upwardly or away from the stationary magnetic plate 84.
In such a conventional example, however, a contact switching mechanism to open or close the contacts 72 and 73 through operation of handle 78 is not separate from the contact breaking mechanisms, such as link 79 and hook segment 73, to disconnect the contacts, but integral with each other. The hook segment 80, when the contacts are to be cut off, disengages from the movable magnetic plate 81 and at once rotates around a shaft 87 and thereafter the movable arm 74 turns. Hence, there has been a problem in that it takes a long time to actually disconnect the contacts 72 and 73 after detection of abnormal load. Also, even when the contacts 72 and 73 are intended to be disconnected with a larger gap therebetween, the gap is restricted because the handle 78 and movable arm 74 are connected through the link 79 and hook segment 80. Furthermore, the contact open-closing mechanism and contact cutoff mechanism in this example are integral with each other, whereby the contact cutoff mechanism is subjected to mechanical stress each time the handle 78 is hand-operated to open or close the contacts, especially subjected to the maximum strength in the transition from open condition to close condition, thereby having created a problem in that the mechanism is liable to be damaged.